1. Field of the Invention
The invention relates to a power-control device for power-output units.
2. Related Technology
In order to protect power amplifiers, especially their high-frequency power transistors, from power overload caused by reflected power from the antenna if the antenna is inadequately matched, it is conventional to monitor the forward and return waves moving on the antenna supply line by means of a directional coupler and if necessary to reduce the input level to the power output units via a control circuit. With transmitters disposed in close spatial proximity, which are operated within the same frequency range, in some cases, with only a slight frequency difference, for example, on ships, narrow-band transmission filters are often inserted into the antenna supply line to avoid an undesirable reduction in transmission power. However, these have the disadvantage that they are extremely complex because of their narrow bandwidth; they are therefore expensive, and the high-frequency switching elements are exposed to extreme stress.
It is already known from DE 39 18 159 A1 that the control circuit can be operated in a frequency-selective manner in order to avoid this narrow-band transmission filter. For this purpose, the forward signal and the return signal obtained from the directional coupler are both subjected to an I/Q demodulation, wherein a reference signal, which is derived from the high-frequency signal branch before the power-output unit to be controlled, is used for the mixer of the I/Q demodulator. The frequency selectivity in this case is dependent upon the edge steepness of the low-pass filters connected downstream of the mixers in the in-phase (I) branch and the quadrature-phase (Q) of the baseband. Relatively stringent requirements must therefore be placed on the low-pass filters, of which a total of four are required (one for the I-branch and Q-branch of the forward signal and the return signal respectively), which can generally only be fulfilled by active filters with relatively complex circuit technology. Moreover, the control signal must be generated in a complex manner by means of a total of four squaring units, two adding units and two root-extractor units. If the alternative suggestion of realising this procedure by means of digital signal processing operating in multiplex mode is taken up, the complexity is still considerable even with this digital realisation.